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Discussion Starter #1
Have asked for help here numerous times - still car must warm up for 10 minutes before I can gas it and not have afm pop or cut out altogether. Have replaced all sensors, cold start valve, thermotime, air regulator. Have a used afm that I put in and am wondering if it's the problem. It's a federal afm and I have a CAlornia 77 280z coupe. Guessing it might be an air problem, but really don''t know. Any ideas- am at my wits end on this.
 

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I don't think it's your AFM, but not 100% sure. Sounds like something is wrong with your intake vacumn.
 

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Have seen your previous posts, but thought the suggestions there would have solved problem. Have you done the following:
1) Checked for vacuum leaks? (Remove oil filler cap at warm idle. Engine should stumble and/or die. If not, you have a vacuum leak).
2) Undone, cleaned, recrimped and reattached EVERY electrical connection under the hood?
3) Checked that you have a good, topped up battery, again with clean, non-corroded connectors?
4) Made sure all fuses (including fusable links) are making good, clean contact?
5) Checked the air bypass valve? (Or is that the same as the air regulator — might be). It's easy, though: take it off, hook 12V to the two connectors and watch the little door close in about a minute. If it doesn't, or if you can't look through it (door stuck closed), the valve is no good, which could cause backfire through AFM.
6) Made sure you're running premium gas?
7) Not done anything immediately previous to this problem starting? If you have, go back and look for wires and vacuum fittings you might have mistakenly dislodged.
8) Made sure you are running NGK spark plugs?
9) Made sure timing is correct?

And just to make sure we're all on the same page, the car runs fine once it does warm up, correct? JUST in the transition period when it's on the cold-start cycle is when you're having the problem, right?

If the car is running all right when warmed up, I doubt if your problem is in the AFM, distributor or anywhere else but the cold-start system. (Although I suspect backfiring could "injure" the AFM.) What I would do at this point is look up the problem in the Factory Service Manual and start following their excellent trouble-shooting 'trees' until the problem was isolated and solved. If you don't have an FSM, well I would suggest getting one — unless you don't mind tearing your hair out and spending lots of money you probably didn't have to. If for some reason you can't get an FSM, I would start by vigorously jiggling wires and connectors that are part of the cold-start system when the car is warming up. You'd be surprised how often this reveals a problem. Jiggle them when it's warm, too. Failing that, disconnect the cold start system entirely. Unless it's freezing cold, the engine should start without that stuff hooked up (mine does), it will just run ragged until warm. However, if it's still backfiring when cold start stuff is not hooked up, then you've eliminated that system as part of the problem. At that point, I might try swapping another ECU (from a wrecking yard car, quit buying so much new stuff) and see if that makes any difference.

My basic philosophy with this stuff is do the quick, cheap stuff first and work your way up. I have a whole drawer full of sensors and misc fittings I've pulled of motors at the junkyard and I think the whole drawerful cost $15 or $20 tops. I try to test most of them (using specs in the Factory Manual, hint, hint) when I get them home, and just throw out the ones that don't pass. I have 5 or 6 good air bypass valves now, for example. They were like a buck or two each, not $60 or whatever MSA gets for new ones these days. If I was really into building the best Z I could, I'd test out components using spares to find the problem. And only THEN, when I was SURE that I'd solved it, would I ever give these robbers top dollar for a shiney new sensor. Just my 2¢.
 

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Discussion Starter #4
Appreciate your in-depth response- JR. It's basically a cold problem, however it also runs poor when I shut it off for 20 minutes or so and try and restart it. It acts like vapor lock then until temp gauge goes back below half. I have replaced virtually the entire cooling system too. Did a pressure check on fuel pump and it showed 34-35 so it must be OK.
 

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Wow, that's a toughie. The engine runs fine, plenty of power, vacuum advance working, all that stuff — when it warms up OR cools down. Right? I have to admit, I'd drop back to Plan B here. (Again, Plan A is the factory service manual, which will narrow your focus down immensely) If I had a parts car, or extra parts such as the AFM, I guess I'd start trying them. I'd start with the ECU and distributor, as the two symptoms are making me wonder about a common source. Then again, it could be two entirely separate problems in two different systems. In particular, check the pickup in the distributor, which might be going bad, the coil, the ballast resistor and all those various wires for any loose or dirty connex. Also try the wiggle-jiggle thing I was talking about — and make sure ALL your contacts are clean, clean, clean. Based on what I've heard and read, about 90% of the problems with these cars are due NOT to failing components — Nissan actually made pretty good stuff — but to simple electrical problems: bad contacts, bad grounds and/or low voltage.
 

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The restart problem is seperate. That is vapor lock. So a search on heat soak or vapor lock to see what was discussed. That problem CAN be cured.

The cold start problem is something else, though.

Have you checked that the connectors are actually plugged in where they are supposed to be? You can accidentally plug the CSV connector to the air regulator. You can have funny symptoms with that. Eventually, the problem goes away, as the engine temperature heats up enough to force the air regulator to close without the electrical signal it normally uses. (BTW, the CSV connector has 4 wires (white I think) going to it's 2 positions. This is part of the thermotime/CSV circuit.

Just a thought. If you use the FSM, you can trace each wire back from the ECU and make sure that everything is hooked up to where it is supposed to be. Don't laugh. When an engine has been out of the car, anything can happen when it's reinstalled. You may not have done it, but you never know.
 
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